There is a growing need for photodetectors which respond to rapidly-changing illumination but ignore relatively large amounts of steady or slowly-changing illumination. This need has been brought about by the development of the light-emitting diode (LED) which, unlike incandescent lamps, can be made to change light output substantially instantaneously in response to rapidly-changing currents but are capable of very little continuous light output. Advantage has been taken of these properties in photodetection systems to modulate the LED current at a high rate and pick up the modulation with a sensitive detector which does not respond to steady sources, or to "pulse" the LED with high-current, short duration pulses, repeated infrequently so as to maintain low average power. In such systems there is need for a photodetector that produces a high signal output to rapidly-changing illumination and as much as possible ignores slowly-changing and steady illumination, while at the same time acting as a two terminal device so that wiring to a sensor of this type can be just as simple as to more elementary sensors such as photo-transistors and photoresistors. In the prior art such elementary sensors can be used to convert the illumination into its electrical analog but require the later separation and relative amplification of the modulation component. This prior arrangement is disadvantageous because the amplifier is generally remote from the signal source and is subjected at its input to all the noise picked up on the signal lead. In addition, the unamplified signal at the sensor will be usually at a high impedance level so that cable losses become severe, particularly for high modulation frequencies or pulsed operation.